1. Field of the Invention
This invention relates to boosters employed to detonate explosive materials, such as are used in mining, construction, and seismic activity, and more specifically, to explosive boosters that effect optimally efficient detonation of such explosive materials. The present invention has particular applicability to the cast primer type of explosive booster.
2. Background Art
In the use of explosives in mining, construction, and seismic research, it is presently preferred to employ as an explosive material a blasting agent which is less sensitive, and accordingly significantly safer to handle and store, than propellants or high explosives. Such a blasting agent suitable for use in the mining industry is ANFO, a mixture of ammonium nitrate and fuel oil. This material resists detonation when exposed to shock or heat of a degree common to the mining environment. It is also relatively inexpensive.
Nevertheless, due to its insensitive nature, a blasting agent can only be detonated in conjunction with a small quantity of a more sensitive or powerful explosive material which is used to initiate the process. Typically, two components are involved in initiating the detonation of an explosive material. The first of these components is directly stimulated from a control device in order to initiate the explosion. Such components include blasting caps and detonating cords. In the former, a highly explosive material is concentrated in a small package at the end of a cable. The cable is capable of communicating an electrical or other type of stimulus to the blasting cap from the detonation control device. A detonating cord, by contrast, is actually a continuous thread of highly explosive material. A detonating cord detonates along its length in a progressive manner, once a stimulus for detonation is applied at one end. Both blasting caps and detonating cords permit safe, remote initiation of explosions, but neither is of itself capable of generating adequate energy to start the detonation of a relatively insensitive blasting agent.
Therefore, a second component in the blast initiating process is interposed between the explosive and the blasting cap or detonating cord. This interposed element of blast initiation is the booster or primer. A booster functions to amplify the energy of a blasting cap or detonating cord into an explosion sizable enough to initiate the detonation of a relatively insensitive explosive material. Boosters are made of high energy materials adequately sensitive to be detonated by a blasting cap or a detonating cord. Having a larger mass and more explosive energy than blasting caps or detonating cords, a booster will upon detonation produce enough energy to initiate explosive reactions in an adjacent explosive material. A booster is thus critical in most successful explosive operations as an intermediary between blasting caps or detonating cords and a relatively insensitive explosive material.
A typical configuration of the elements of an explosive detonation used in mining, construction, or seismic research is shown in FIG. 1. There a borehole 10 has been drilled to a preselected depth in a rock formation 12 which is to be shattered by explosives, possibly to prepare it for subsequent mechanical removal. A primer or booster 14 has been lowered to the end 16 of borehole 10. By way of illustration, operably engaged with booster 14 is a blasting cap 18 at the end of an electrical conductor 20 which leads to a detonation box 22 or other appropriate detonation control device. With booster 14 and blasting cap 18 thus disposed at the bottom 16 of borehole 10, a suitable blasting agent 24 has been poured into borehole 10 contacting booster 14.
Operation of detonating box 22 will set off blasting cap 18 which in turn detonates primer 14. This detonation releases energy adequate to initiate detonation of blasting agent 24. The entire process is completed within a few milliseconds. In order to contain and drive laterally into rock formation 12 the explosive force of blasting agent 24, the open end 26 of borehole 10 has been stemmed with backfill 28.
Rock formation 12 in which borehole 10 was drilled and equipped for explosive detonation as shown in FIG. 1 could have been at the surface of the ground, at the bottom of a mining pit, or underground at the working face of a mine. Typically an array of boreholes, such as borehole 10, is prepared together in a rock formation before any detonation occurs. Then the columns of blasting agent in the borehole matrix are detonated simultaneously or in a nearly simultaneous or patterned progression of detonations according to the specific consequences sought. The depth of borehole 10 and the height of the column of blasting agent 24 placed therein are dictated by the nature of rock formation 12, as well as the objectives of the blasting exercise.
Since the late nineteenth century, boosters used for the purposes of the initiating explosions have been nitroglycerine products. By accident of circumstances, the shape of these highly explosive products mirrored the surrounding boreholes in which they were most commonly used. As a result they were shaped into elongated cylinders, typically two inches in diameter by eight inches in length or five inches in diameter by twenty-five inches in length.
In the late 1950's and early 1960's, new powerful booster materials were developed which could be cast into various shapes. The boosters into which these materials were made were termed cast primers, because of the method of their manufacture. Cast primers continued, however, to be produced in the traditional elongated cylindrical shape into which boosters had previously been formed. A typical cast primer weighing approximately one pound is two inches in diameter and five inches in length. A common cast primer composition available under the trade name SuperPrime.RTM. is currently marketed by the Trojan Corporation. SuperPrime.RTM. is comprised of Pentolite, a mixture of PETN and TNT.
FIG. 2A shows a cross-section of a booster 30 having such a traditional elongated cylindrical shape. Booster 30 has sides 32 of height H which is usually substantially greater than the diameter D of congruent circular top end 34 and bottom end 36. Formed in booster 30 is a longitudinally disposed passageway 38 traversing the height H of booster 30 between top end 34 and bottom end 36 thereof. In addition, a dead-end passageway 40 is formed in booster 30 parallel to passageway 38 and opening onto bottom end 36 exclusively.
Passageway 38 and dead-end passageway 40 cooperate to receive a means for detonating booster 30. As shown by way of example in FIG. 2A, a blasting cap 42 has been installed in dead-in passageway 40 with its associated conductor 44 emerging from booster 30 at top end 34 thereof through passageway 38.
Explosive boosters capable of housing a dead-end passageway, such as dead-end passageway 40, are termed high-profile boosters. The properties of the material of which a booster is fabricated and purpose to which the booster is applied are factors that determine how short a high-profile booster of that material can be. Generally, the height H of high-profile boosters ranges upwardly from a minimum of 4.5 inches.
Boosters, such as booster 30, are normally installed in boreholes with the sides 32 thereof parallel to the sides of the borehole. Top end 34 is directed toward and in contact with the explosive material which the booster is intended to detonate. Top end 34 of booster 30, in contrast with sides 32 thereof, functions as the primary surface of booster 30 that interfaces with the explosive material 24. As used herein the term "interface surface" will be employed to refer to the primary surface of a booster that would customarily be installed directed toward and in contact with the explosive material to be detonated.
FIG. 2B depicts a low-profile booster 50 having sides 52 of height H and symmetric circular top end 54 and bottom end 56 of diameter D. While in FIG. 2B, booster 50 is depicted as having a height H less than diameter D, it is not necessarily the relative relationship of these two dimensions which determines whether or not a booster is considered low-profile. Rather, as discussed above, it is the properties of the material of which the booster is made and the purpose for which the booster is used that ultimately determine whether a booster of a given height H must be low-profile.
Lacking dead-end passageways, such as dead-end passageway 40 in FIG. 2A, low-profile boosters cannot operably engage a blasting cap, but can be used only in conjunction with detonating cords. Booster 50, being a low-profile booster, is shown as including only a single passageway 57 longitudinally disposed therein between top end 54 and bottom end 56. Either top end 54 or bottom end 56 of low-profile booster 50 could be used as an interface surface. The installation of a blasting cord 58 in passageway 57 with a retaining knot 59 at bottom end 56 of booster 50 would commonly result, however, in top end 54 being the interface surface for booster 50.
The need to employ detonating cords with low-profile boosters severely limits the circumstances in which they can be used. Low-profile boosters continue, however, to mirror the shape of the boreholes in which they are commonly used, as in the ultimate analysis, even with their truncated heights, low-profile boosters are cylindrical in shape.
The cylindrical shape in boosters continues to be in evidence in the hybrid booster 60 shown in FIG. 2C. Booster 60 is comprised of a cylindrical portion 62, reminiscent of a low-profile booster, joined to a high-profile cylindrical portion 64. A longitudinally disposed passageway is formed in booster 60 between circular top end 68 of diameter D and small bottom end 70. Low-profile portion 62 and high-profile portion 64 together, however, have a combined height H which is large enough to permit the formation in booster 60 of a dead-end passageway 72 suitable for receiving a blasting cap in operable engagement with booster 60. The interface surface for booster 60 would correspond under normal usage to top surface 68.
Hybrid boosters, such as booster 60, retain the unlimited utility of high-profile boosters, but they are plagued by difficulties relating to their method of manufacture, which necessitates roughly twice the manufacturing steps required to make traditional single-diameter cylindrical booster. Hybrid boosters have accordingly been perceived as overly expensive in relation to any benefits otherwise derivable therefrom.
The energy generated by the detonation of a booster travels outwardly therefrom in the form of a shockwave front which is intended to enter an explosive material and propagates therethrough. The shockwave front itself produces a corresponding traveling region of local compression of the explosive material. Compression creates conditions in which the chemical decomposition of the explosive material into gases can occur. Therefore, behind any adequately intense shockwave front passing through an explosive material is a region of expanding gases in which explosion is taking place. The boundary between the compression region and the explosive region is the detonation wave front of the explosion, which also travels through the explosive material as detonation progresses.
The detonation wave front for any given explosion has a velocity which varies with time over the nonetheless short duration of that explosion. As the detonation wave front is a moving wave front, this means that temporal variations in detonation wave velocity can simultaneously be described as variations correlated to the position of the detonation wave front in the exploding material. A common point of reference for this spatial aspect of detonation wave front velocity variation is the distance from the interface surface or top of the booster that initiated the explosion. The detonation wave front velocity in an explosive material is affected by the nature of that material, the shape in which the material is confined, and the intensity as well as shape of the shockwave front originally projected thereinto from a booster.
Each type of explosive material has a characteristic optimum detonating wave front velocity at which that explosive material decomposes in an ideal manner. At this detonating wave front velocity the maximum possible energy is released in explosive form from each portion of the explosive material through which the detonating wave front travels. This optimum velocity is the steady-state velocity for the explosive material involved. In theory, it is the velocity at which a detonating wave front in a particular explosive material constrained in a particular shape will tend to travel in the long run, once detonation has been initiated. Velocities of a detonation wave front that are either greater than or less than the steady-state velocity indicate that less than the full potential explosive energy in the explosive material is being released by the explosion process. In this light, detonation wave front velocity at each point in a charge of exploding material may be taken as an indicator of the quality of the reaction of the chemicals of that material at each specific location therein.
The actual velocity of the detonating wave front in an explosive material can vary dramatically over the course of an explosion. This is particularly true in the region of the explosive material close to the booster that has initiated the explosion. If the velocity of the detonating wave front initiated in the explosive material by the booster is less than the steady-state velocity, the explosion is termed an under-driven detonation. Typically, the velocity of the detonating wave front in an under-driven detonation will gradually rise toward the steady-state velocity as the detonating wave front propagates through the explosive material and the chemical reactions therein drive the rate of reaction and the velocity of the detonating wave front toward an optimum state of product decomposition at the steady-state velocity.
Detonations in which the velocity of the detonating wave front in the explosive material close to the booster is greater than the steady-state velocity for that explosive material, are called over-driven detonations. In these, the velocity of the detonating wave front will diminish, approaching the steady-state velocity as the detonating wave front travels through the explosive material away from the booster. Occasionally this drop in velocity is so abrupt that the velocity of the detonation wave front falls below the steady-state velocity. Gradually, the detonation wave front velocity will thereafter rise until the steady-state velocity is once again achieved. These detonations will generally be considered to under-driven explosions.
In an under-driven detonation, the distance from the interface surface or top of the booster at which the velocity of the detonation wave front reaches the steady-state velocity is termed the run-up distance for that detonation. An efficient detonation requires that the steady-state velocity be achieved as promptly as possible. In terms of the efficient consumption of explosive material, detonating wave front velocities of the under-driven variety of detonation represent a loss of potential explosive power. Accordingly, for the designer of an efficient detonation, the minimizing of the run-up distance is an important objective.
In an over-driven detonation, the distance from the interface surface or top of the booster at which the velocity of the detonation wave front slows to and assumes the steady-state velocity is termed the transient velocity distance. Minimizing the transient velocity distance is not necessarily an objective of the designer of an efficient detonation, as enhanced shattering action in the immediate area of the booster is achieved in over-driven detonations. This in turn may render more effective the explosive pressure developed in subsequent stages of the explosion.
Accordingly, the overall efficiency of an explosion can be evaluated in terms of whether the detonation is under-driven or over-driven, the time following booster detonation at which steady-state velocity is achieved, and the degree to which that velocity is maintained throughout the balance of the explosion thereafter. These parameters of an explosive detonation will be illustrated through the use of the graphs of FIGS. 3A, 3B, and 3C and 4A and 4B, which contain velocity traces for explosions detonated by the various cylindrical boosters shown in FIGS. 2A, 2B, and 2C and already discussed.
FIGS. 3A, 3B, and 3C are examples of velocity traces resulting from the use of various sizes of nitroglycerine boosters of the traditional elongated cylindrical shape, such as booster 30 of FIG. 2A, in a six-inch diameter borehole to detonate a charge of ANFO. ANFO has a steady-state velocity under those conditions of approximately 12,000 feet per second. All of the detonations illustrated in FIGS. 3A, 3B, and 3C were under-driven.
FIG. 3A illustrates the velocity trace of a 1.25 pound booster, such as booster 30 of FIG. 2A, having a height of eight inches and a circular diameter of two inches. The detonation wave front velocity in the vicinity of the interface surface at the top of the booster can be seen to have been substantially less than the steady-state velocity for the material being detonated. For the portion of the velocity trace shown in FIG. 3A, the detonation wave front velocity never did in fact reach the steady-state velocity for ANFO under the conditions present. Under most circumstances, this would suggest that a booster had been used which was not adequately large in relation to the energy level of its constituent material for the size of borehole and type of explosive material detonated.
FIG. 3B illustrates the velocity trace produced by a larger 2.75 pound booster, such as booster 30 of FIG. 2A, having a height of eight inches and a circular diameter of three inches. As in FIG. 3A, the detonation illustrated in FIG. 3B was under-driven. Nevertheless, the resulting velocity trace reveals that the detonation wave front velocity increased rapidly enough that it eventually reached the steady-state velocity at a run-up distance of approximately 21-23 inches. The rapid rise of the detonation wave front velocity illustrated in FIG. 3B would under most circumstances be taken as an indication that the detonation illustrated was more efficient than that of FIG. 3A.
FIG. 3C shows the velocity trace resulting from the use of yet a larger six pound booster, such as booster 30 of FIG. 2A, which was six inches in diameter and five inches in height. The additional energy provided by the larger booster is seen to have resulted in a shortened run-up distance and in enhanced detonation wave front velocities, even where these were less than the steady-state velocity for ANFO under the conditions present. In the case illustrated in FIG. 3C, the diameter of the booster employed was substantially equal to the diameter of the borehole in which it was detonated. Prior to the present invention, conventional wisdom was to the effect that such was the optimum desirable relationship between booster diameter and borehole diameter, if maximally efficient detonation were an objective. FIGS. 4A and 4B are examples of velocity traces of various other cylindrically shaped boosters, such as the low-profile booster 50 of FIG. 2B and the hybrid booster of FIG. 2C. In each instance,the booster involved was made of Pentolite and used in a ten-inch diameter borehole to detonate a charge of ANFO. ANFO has a steady-state velocity under those conditions of approximately 14,000 feet per second.
FIG. 4A shows the velocity trace for a five-pound low-profile booster, such as booster 50 of FIG. 2A. In the immediate vicinity of the booster, the detonation wave front velocity exceeded the steady-state velocity for the explosive material being detonated. The detonation wave front velocity dropped abruptly, however, and for a substantial distance from the top of the booster was less than the steady-state velocity before it increased to that optimum level. The detonation is thus considered under-driven, and in the case shown in FIG. 4A the run-up distance for the detonation was approximately 24 inches.
A velocity trace for a hybrid booster, such as booster 60, weighing three pounds is shown in FIG. 4B. The detonation that resulted was over-driven, as the detonation wave front velocity did not fall below the steady-state velocity to any substantial degree or for any appreciable period. The transient velocity distance shown of approximately 20 inches would suggest that enhanced shattering action occurred in the immediate area of the booster with corresponding favorable effects on detonation efficiency.
As is readily appreciable from the velocity traces discussed above, the character of the booster used to detonate an explosive material can have a significant impact upon the quality of explosion that results. Enhanced detonation efficiency will predictably result in the need to employ a smaller quantity of explosive material for equivalent results. Thus, while a booster represents but a small percentage of the total cost of preparing for a explosion, manipulation of the type of booster used offers the potential for large increases in the overall efficiency of the detonation at a small change in its total cost. With this objective in mind, research was commenced to determine on a scientific basis the best suited booster for each varying borehole condition. It was known that changing the composition of a booster would affect the nature of the detonation that it produced. Apart then from this variable, the object was to maximize the release of energy in the blasting agent employed by manipulating the size, shape, and orientation of the booster employed to initiate its detonation.
Prior to the present invention no single booster had been devised which resulted in minimal weight, optimal detonation efficiency, unlimited functionality due to the capacity to employ blasting caps, and reasonably acceptable manufacturing costs.